Conventionally, a cubic boron nitride (cBN) sinter used as a cutting tool or a wear-resistant tool contains therein a sintering agent or a binding agent such as TiN, TiC and Co. The sinter is obtained by sintering cBN powder with the sintering agent or the binding agent at a pressure of about 4 to 5 GPa. The sinter contains therein the binding agent at about 10 to 40%. The binding agent greatly affects the strength, the heat resistance and the heat diffusion of the sinter, and especially in cutting a ferrous material, defects and cracks are likely to occur on the cutting edge, which shortens the lifetime of the tool.
As an approach to prolong the lifetime of the tool, there is known a manufacturing method for a cBN sinter without using a binding agent. In the method, hexagonal boron nitride (hBN) and a catalyst such as boron nitride magnesium are used as a starting material, and they are sintered and reacted. According to the method, since no binding agent is used, the cBN individuals bind to each other strongly and the thermal conductivity becomes as high as 6 to 7 W/cm° C. Thus, the cBN sinter is used as a heat sink material or in a TAB (Tape Automated Bonding) tool or the like. However, since a certain portion of the catalyst will remain in the sinter, and when the sinter is heated, due to the thermal expansion difference between cBN and the catalyst, it is easy for fine cracks to occur. Thereby, the heat-resistant temperature is as low as about 700° C., which would be a big problem for a cutting tool. Further, since the grain size is roughly as large as 10 μm, it improves the thermal conductivity but weakens the strength thereof, making it intolerable to a large cutting load.
In general, the cBN sinter used as a cutting tool is manufactured by sintering cBN powder with a binding agent such as TiN, TiC and Co at a pressure of about 4 to 5 GPa. The manufactured sinter contains therein the binding agent at about 10 to 40%, and the binding agent deteriorates the strength, the heat resistance and the heat diffusion of the sinter. Thereby, in high-speed cutting, and especially in cutting a ferrous material, defects and cracks are likely to occur on the cutting edge, which shortens the lifetime of the tool.
As an approach to solve this problem, there is known a manufacturing method for a cBN sinter without using a binding agent. In the method, hexagonal boron nitride (hBN) and a catalyst such as boron nitride magnesium are used as a starting material, and they are sintered and reacted. According to the method, since no binding agent is used, the cBN individuals bind to each other strongly and the thermal conductivity becomes as high as 6 to 7 W/cm° C. Thus, the cBN sinter is used in as a heat sink material or in a TAB (Tape Automated Bonding) tool or the like. However, since a certain portion of the catalyst will remain in the sinter, and when the sinter is heated, due to the thermal expansion difference between cBN and the catalyst, it is easy for fine cracks to occur. Thereby, the heat-resistant temperature is as low as about 700° C., which would be a big problem for a cutting tool. Further, since the grain size is roughly as large as 10 μm, it improves the thermal conductivity but weakens the strength thereof, making it intolerable to a large cutting load.
On the other hand, the cBN sinter can also be obtained through direct conversion by sintering an atmospheric BN such as hBN at ultra-high pressure and temperature. For example, such method of obtaining a cBN sinter by converting hBN to cBN at ultra-high pressure and temperature is disclosed in Japanese Patent Laying-Open No. 47-34099 (PTD 1) and Japanese Patent Laying-Open No. 3-159964 (PTD 2).
Further, there is a method of using pyrolytic boron nitride (pBN) as a starting material to obtain a cBN sinter, which is for example disclosed in Japanese Patent Publication No. 63-394 (PTD 3) and Japanese Patent Laying-Open No. 8-47801 (PTD 4). However, in this method, strict conditions of 7 GPa and 2100° C. or more are required.
Another method of obtaining a cBN sinter at a pressure of 6 GPa and a temperature of 1100° C. which are relatively mild in comparison to the above conditions is disclosed in Japanese Patent Publication No. 49-27518 (PTD 5). In this method, since the grains of hBN serving as the raw material is 3 μm or less, hBN contains boron oxide impurities and adsorption gas at several percentage. Due to the influence of the impurities and the adsorption gas, the sintering will proceed insufficiently or the hardness will decrease due to the presence of oxides, which makes it impossible to be used as a cutting tool and a wear-resistant tool.
In order to solve the above problems, Japanese Patent Laying-Open No. 11-246271 (PTD 6) discloses a method of using low crystalline hexagonal boron nitride as a starting material to synthesize a cBN sinter at 6 to 7 GPa and 1550 to 2100° C.